The term display device broadly refers to any device which is capable of reproducing images. Such devices are commonly used in televisions for regenerating images from radio waves or in video-display terminals for reproducing images generated by a computer. In addition, display devices could be incorporated with a communication device, such as a telephone, for displaying choices of telephone services which may be selected by the user.
Most display devices in use are of the cathode-ray tube (CRT) type 10 shown in FIG. 1. The cathode-ray tube 10 is an elongated vacuum chamber having a cathode 12 at one end and a cathodoluminescent phosphor screen 14 at the other end. In a typical CRT 10, the cathode 12 emits a beam of electrons 16 that is deflected by a deflection coil 18 in a raster scan over the phosphor screen 14. The phosphor screen 14 converts part of the electron energy into light. CRT's have a good color and grey scale range. Furthermore, the brightness of the image produced on the screen looks about the same from any viewing angle. The principle disadvantage of the CRT is its bulky glass envelope which must be long to allow the emitted electron beam to be deflected over the entire screen. Furthermore, the glass envelope must be strong enough to prevent the weight of the atmosphere outside from crushing the tube or otherwise filling the vacuum inside.
Flat-panel liquid-crystal display (LCD) devices have become more readily available on the market, particularly in lap-top computers and portable televisions for which CRTs are impractical. Generally, LCDs include a liquid-crystal modulator which modulates a polarization-encoded image onto a linearly polarized input light beam. The polarization-encoded image may then be revealed by an analyzer, e.g., a polarizer.
Almost all LCDs can trace their origin to the twisted-nematic display which has a liquid-crystal modulator made with a nematic liquid-crystal substance. Nematic liquid-crystals have just one unliquid-like property; their elongated molecules prefer to be aligned with one another. These aligned molecules can be made to twist relative to one another by a predetermined amount, thereby forming a helical structure referred to as a twisted-nematic (TN).
A TN liquid-crystal modulator 100 is shown in FIG. 2(a). As shown, the TN liquid-crystal material 110 is disposed between two flat substrates 112 and 114 covered by alignment layers. The two alignment layers are specially designed so that the molecules 110-1 and 110-2 of the liquid-crystal material 110 tend to align with a particular direction 122 or 124 of the alignment layers 112 and 114, respectively. The alignment layers 112 and 114 have directions 122 and 124 which are orthogonally separated by a 90.degree. angle. In such a case, the liquid-crystal material 110 forms a TN structure which is anchored on both substrates 112 and 114, as depicted.
If light, which is polarized in a direction 126 in the plane of the alignment layers, passes through the liquid-crystal modulator 100, its polarization 125 will be rotated by 90.degree. by the TN structure provided that: EQU .DELTA.n.multidot.d&gt;&gt;.lambda..sub.light ( 1)
where .DELTA.n is the difference between the extraordinary and the ordinary indices of refraction (n.sub.ext -n.sub.ord) of the liquid-crystal material 110, d is the thickness of the modulator and .lambda..sub.light is the wavelength of the polarized light.
If a few volts are applied between the two substrates 112 and 114, the molecules of the liquid-crystal material align with the electric field, as shown in FIG. 2(b). The polarization of light 125 passing through the modulator 100 would be unchanged. The modulated light subsequently passes through an analyzer 130, which transmits light polarized in a particular direction 128, e.g., the same direction as the direction 126 of the polarized light before it passes through the modulator 100. If the light exits the modulator 100 with the same polarization direction 128 as the analyzer 130, it is transmitted by the analyzer 130. The modulator 100 is said to be in an "on" state. If the light exits the modulator 100 with the other polarization, i.e., with a polarization direction 124 at right angles to the polarization direction of the analyzer 130, it is blocked by the analyzer 130. The modulator is then said to be in an "off" state.
Typically, two polarizers are used in a TN-LCD device, namely, a polarizer which produces, from an unpolarized light source, the initial polarized light with the direction 126 that is incident on the liquid-crystal modulator 100 of the TN-LCD device and the analyzer 130.
Illustratively, as shown in FIG. 3, the modulator 100 is divided into picture elements or pixels 140 by forming a linear array of transparent (e.g., indium tin oxide or ITO) conductors 150-1, 150-2, . . . , 150-I and 160-1, 160-2, . . . , 160-J on each respective substrate 112 and 114 under the alignment layers, so that the conductors under one layer (e.g., covering the substrate 112) are orthogonal to the conductors under the other layer (e.g., covering the substrate 114). Pixels are formed in regions where the orthogonal conductors (e.g., 150-1 and 160-1) under the two alignment layers cross. The absence or presence of an electric field applied to a pixel 140 determines the response of the pixel 140 and thus whether the pixel 140 will appear dark or light when viewed through the analyzer 130 (FIG. 2(b)). Select voltages are sequentially applied to the pixel row conductors (e.g., the conductors 150-1, 150-2, . . . , 150-I) one at a time. Column voltages are applied to the column conductors (e.g., the conductors 160-1, 160-2, . . . , 160-J) depending on whether the pixel in that column is on or off for a given row.
As shown in FIG. 3, a single longitudinal column electrode (e.g., 160-1, 160-2, . . . , 160-J) is used for each pixel of a particular column. Thus, when a voltage is applied to any pixel via its respective row and column conductors, all of the pixels in the particular column of that pixel will experience a voltage, albeit, not as strong as the voltage across the pixel associated with the row conductor to which a voltage is applied. Such extraneous voltages increase the field on a pixel that is supposed to remain off. For this reason, the pixelated TN-LCD modulator 100 is limited in the number of rows which can be displayed. In a frame time F, N rows are displayed, each during a row-time period p (F=Np). The liquid crystal responds to the rms (root mean square) voltage applied to it. However, when the select voltage is applied with a duty cycle of only 1/N, it is hard to achieve a large enough ratio of V.sub.on.sup.rms to V.sub.off.sup.rms. This is partly because the TN liquid crystal does not change between the two states shown in FIGS. 2a and 2b over a sufficiently small range in voltage. Thus, for a TN structure, the number of rows N which can be displayed on a TN-LCD is less than 100. Furthermore, in order to construct a TN-LCD capable of displaying that many rows, the electro-optic response of the liquid-crystal modulator is compromised so that the "on" and "off" states of the pixels are no longer in ideal alignment with the electric field and 90.degree. twist, respectively. As such, the contrast ratio for light traveling in some directions is reduced from what can be achieved with a continuously applied voltage wave form, thereby reducing the viewing-angle range.
It is disadvantageous to apply a constant DC voltage to the liquid-crystal modulator as this tends to break down the liquid-crystal therein. Therefore, the polarity of the applied voltage is reversed periodically to cancel the DC component.
An improved LCD called a supertwist nematic LCD (or STN-LCD) is available in which the twist angle of the modulator is increased from 90.degree. to between 200.degree. and 270.degree.. STN-LCDs permit displays with 200 to 240 rows thus making popular 640.times.480 display devices possible (e.g., using two adjacent STN-LCDs of 240 rows each on the same glass plate substrates). STN-LCDs are disadvantageous because they are slow. A STN liquid-crystal modulator, to which optimum voltages are applied, can have a transmission response which decays quickly after a voltage is applied. But, this also causes the pixels to "relax" from the bright or "on" state to the dark or "off" state in between frames, thereby reducing brightness and contrast. However, STN liquid-crystal displays are usually designed so that their response does not decay rapidly in between the application of voltages (i.e., in between frames). Such STN-LCDs must be driven with "on" state and "off" state rms voltages with a low duty cycle. The net result is an STN-LCD which is slow; i.e., moving images often disappear from the screen in an effect called "submarining". This makes it difficult to implement a "mouse" or trackball pointer. Furthermore, grey scales (and thus full color) can only be implemented with spatial or temporal dithering. In spatial dithering, the pixels of the modulator are treated as sub-pixels which are grouped together to form pixels of the display. To display a grey level, none, all, or some of the sub-pixels grouped to form a pixel of the display are turned on depending on the intensity of the pixel of the display. Spatial dithering is disadvantageous because resolution is sacrificed in order to display grey levels. In temporal dithering, the "on" voltage of a pixel is varied over a number of frames, depending on the pixel's intensity, to produce an rms value intermediate between V.sub.on.sup.rms and V.sub.off.sup.rms. Temporal dithering is disadvantageous because it leads to flicker in the displayed pixelated image that can be detected by the human eye.
An alternative to the STN-LCD is shown in FIG. 4 called the active-matrix LCD, or AMLCD 200. In the AMLCD 200, a TN liquid-crystal material 210 is used as before. One common electrode 221 is formed under one alignment layer 212 and a two-dimensional array of electrodes 251, 252, 253, 254, 255, 256, 257, 258, 259 (i.e., one for each pixel) is formed under the other alignment layer 214. Furthermore, an active element such as a thin-film transistor or diode (e.g., the transistor 271) is provided for each of the individual electrodes 251-259 of the array. Conductors 281, 282, 283, 284, 285, 286 are provided for each row and column of the matrix, with the gate of each transistor (e.g., the gate 272 of the transistor 271) connected to a corresponding row conductor (e.g., the conductor 284), the source of each transistor (e.g., the source 273) connected to a corresponding column conductor (e.g., the conductor 281) and the drain (e.g., the drain 274) connected to the corresponding pixel electrode (e.g., the electrode 251). When appropriate voltages are applied to the row and column to which a transistor is connected (e.g., the conductors 281 and 284), a voltage appears at the electrode of the pixel (e.g., the electrode 251) which charges a capacitance between the pixel electrode and the common electrode (e.g., the electrodes 251 and 221, respectively). This charge remains until the next time the appropriate charges are applied to the transistor of the pixel. Thus, unlike the STN-LCDs, it is not necessary to use a low duty-cycle drive voltage. The voltage applied to a pixel is usually inverted in succeeding frames.
AMLCDs offer several advantages including superior grey scale to the STN-LCD and the ability to display full-color images. It is also possible to speed up the liquid crystal without affecting frame-response. However, the brightness of the display suffers somewhat because a portion of each pixel is blocked by the opaque layers that form the transistor and the conductors connected thereto. Moreover, in order to construct an AMLCD, a very large integrated circuit having a transistor for each pixel must be fabricated. Thus, the cost of an AMLCD is approximately four times that of an STN-LCD.
An active-addressing solution for STN-LCDs has also been proposed. In such a solution, a faster liquid-crystal material is used in the modulator. In order to overcome the problems associated with the rapid decay of the response of the pixels, each pixel is refreshed several times in one frame. To that end, a set of orthogonal voltage waveforms are applied to several rows at the same time.
The active-addressing solution would reduce the "submarining" effect. However, it is still uncertain if an effective range of grey scales can be provided. Furthermore, the driver circuit is much more complicated because it must generate the orthogonal voltage waveforms and analog column voltages. The driver circuit must calculate the analog column voltages from the orthogonal functions and the pixel information of all the rows at high speed.
In an alternative to using nematic liquid-crystals, a display system has been proposed which uses ferroelectric liquid-crystals. See J. Kanbe, "FLCDs Offer Many Desirable Characteristics" Display Devices 1992, P. 18-20; A. Tsuboyama, Y. Hanyu, S. Yoshihara & J. Kanbe, "S3-1 Invited Characteristics of Large Size, High Resolution FLCD" Japan Display p. 53-56 (1992). Ferroelectric liquid-crystals exist in a smectic C* state. In this state, the molecules tend to line up in layers as shown in FIG. 5(a). In the bulk smectic C* state, the molecules are oriented on a cone of angle .theta. as shown in the center of FIG. 5(a) and with greater clarity in FIG. 5(b). The relative angular position of the molecules on this cone rotates by a fixed amount from layer to layer. Near a surface of the substrates 312, 314 in FIG. 5(a), the molecules still line up in layers and lie on the surface of the cone, but are forced to choose one of the two positions on the cone which are also parallel to the substrate.
In the exemplary ferroelectric LCD (FLCD) shown in FIG. 5(c), a modulator 300 is provided in which the alignment layers have the same direction 322 and 324. Also, the two substrates are brought close together so that a thin liquid-crystal layer is formed between the two substrates 312, 314. The molecules therefore tend to line up in stacked layers as shown near the substrates in FIG. 5(a). As shown in FIG. 5(b), if an electric field is applied to the liquid-crystal modulator 300 in the direction of the axis through the points A and B, the molecules may be pulled by a dipole moment thereof so that they lie at a particular location C on the cone. Similarly, if an opposite-polarity electric field is applied, the molecules can be pulled so that they lie at an opposite location D of the cone. In either case, the dipole moment per unit volume (polarization) P of the liquid crystal material aligns with the applied electric field.
As shown in FIG. 5(e), if a light ray polarized in the direction 301 is directed through the modulator perpendicularly to the alignment of the molecules, an ordinary ray emerges which is polarized in the same direction 302 as the incident ray. If, however, by applying a voltage to the modulator, the molecules can be oriented at a 45.degree. angle to the polarized light, then both an extraordinary and an ordinary ray are obtained. One of the rays has a phase shift with respect to the other ray and thus the emergent combined light ray could have its polarization rotated by 90.degree. if the layer has the right thickness and the phase shift is 180.degree..
It is necessary to reduce the thickness of the liquid-crystal modulator 300 so that the molecules can only lie in one of two directions a or b in the plane of the layers separated by the angle 2.theta. as shown in FIG. 5(c). It is necessary to reduce the thickness even further to, for example, 1.5 .mu.m, so that a 180.degree. phase shift between the ordinary and extraordinary rays occurs. The two directions result from a tendency of the molecules to lie on the surface of the cone and to lie in the plane of the alignment layers. Such a liquid-crystal modulator is advantageous because it has a "memory". In other words, if pulled by a voltage in a particular one of the two directions, a or b, the molecules tend to stay in that direction for some time after the voltage is removed unless pulled into the other direction by an opposite voltage.
A flat-panel display 400 using active row-backlights and LCD column shutters is shown in FIG. 5(d). See U.S. Pat. Nos. 5,083,120, 4,924,215; T. Nelson, M. Anadan, J. Mann & E. Berry "Leaky Lightguide/LED Row-Backlight, Column-Shutter Display" IEEE Transactions on Electron Devices, vol. 38, no. 11, p 2567-69 (1991); T. Nelson, J. Patel & P. Ngo, "Row-Backlight, Column-Shutter Display Concept" Applied Physics Letters, vol. 52, no. 13, March 1988, p. 1034-36; T. Nelson, J. Patel, "Row-Backlight, Column-Shutter Display: A New Display Format" Displays, April, 1989 p. 76-80. Illustratively, the display 400 has an active backlight 410 formed by a number of elongated leaky light guides 411 arranged in parallel rows. Each row 411 is alternately illuminated one row at a time for a row-time interval. The light from these rows is polarized in a particular direction 401 and applied to a liquid-crystal modulator 420 which preferably is made with a ferroelectric liquid-crystal material. Thereafter, the polarization-encoded light beam produced by the liquid-crystal modulator is then revealed by an analyzer 440 which transmits light polarized in the direction 402.
The liquid-crystal modulator 420 has one common electrode 431 formed under one alignment layer 430. The liquid-crystal modulator 420 also has a number of column electrodes 421 formed under the other alignment layer 422, each of which defines a column shutter of the liquid-crystal modulator 420. A pixel is defined by the intersection of a column electrode 421 and a row backlight 411. As before, a voltage is applied between the column electrodes 421 and the common electrode 431 for each row of light depending on whether the corresponding pixel is to be on or off. The voltage applied between each column electrode 421 and the common electrode 431 controls the state, i.e., "on" or "off", of the corresponding column shutter of the liquid-crystal modulator 420.
The active row backlight, column shutter display can produce a multitude of grey scales and hence full color without wasting any light. To produce grey scales, the prior art teaches a pulse-width modulation method in which the column shutters are in the "on" state for only a fraction of the row-interval in which a row of light is transmitted. The state of the column shutters is changed to the "off" state during the row-interval. However, the prior art also teaches that the shutters are changed from the "off" state to the "on" state before the start of the next row-interval to prepare the shutter for the next row of light. Thus, the minimum time for displaying a pixel equals the time required to make two transitions. Viewed another way, a row period may be defined as the time from the beginning of one row-interval when one row backlight becomes active to the beginning of the next row-interval when the next row backlight becomes active. Two column-shutter transitions (i.e., from "off" to "on" and from "on" to "off") are required in each row-period.
This presents a problem for providing higher resolution or full-color displays. Moreover, in order to produce color in a display, it is necessary to provide, for each row of pixels in the display, one row backlight for each of the colors red, blue and green (e.g., by providing red, blue and green leaky lightguides). It is also possible to provide separately driven red, blue, and green sources to each leaky lightguide and to operate them at different times. In either case, because the frame time should be fixed to avoid flicker, the column shutters must respond, i.e., be able to change from "off" to "on" and from "on" to "off", three times as fast for a given resolution. However, the response time of ferroelectric liquid-crystals cannot easily be increased to this speed if two transitions per row backlight are required.
It is therefore an object of the present invention to provide an LCD flat-panel display device which can produce full color and grey scales. In particular, it is an object of the present invention to provide an active row-backlight, ferroelectric LCD column-shutter display device in which the column shutters need only change states once per row backlight per color per frame.